1. Field of the Invention
The present invention relates to a sheet conveyance apparatus capable of correcting skew of a sheet while conveying the sheet.
2. Description of the Related Art
An image forming apparatus includes a sheet conveyance apparatus configured to correct skew of a sheet by conveying a plurality of sheets speedily and successively along a reference surface while maintaining a short interval between a trailing edge of a preceding sheet and a leading edge of a following sheet.
As discussed in Japanese Patent Application Laid-Open No. 11-189355, an electrophotographic image forming apparatus includes a sheet conveyance apparatus disposed on the upstream side of a toner image transfer unit. A reference member, positioned near a sheet conveyance path, has a reference surface parallel to a conveyance direction of sheets.
Two or more skew rollers, located close to the reference surface, obliquely convey a sheet toward the reference surface. When the sheet collides with the reference surface, the sheet rotates to correct its orientation and starts moving straight along the reference surface. A positioning roller, located on the downstream side of the reference surface, is movable in a direction perpendicular to the sheet conveyance direction. When the positioning roller conveys a skew-corrected sheet in a direction perpendicular to the sheet conveyance direction, the sheet reaches a predetermined set position.
As discussed in Japanese Patent Application Laid-Open No. 2003-146489, an electrophotographic image forming apparatus includes a sheet conveyance apparatus disposed near a conveyance path used for reverse surface printing. The sheet conveyance apparatus includes a first skew roller positioned close to a reference surface and a second skew roller disposed on the upstream side of the first skew roller and positioned far from the reference surface. A skew amount set for the second skew roller is comparable to that for the first skew roller. Therefore, when the first skew roller starts conveying a sheet, the sheet does not rotate around its centroid.
A sheet conveyance apparatus discussed in Japanese Patent Application Laid-Open No. 2005-104712 includes a first skew roller and a second skew roller positioned on the upstream side of the first skew roller. The first skew roller contacts a surface of a sheet at a position closer to a reference surface than the centerline of a sheet parallel to the sheet conveyance direction. The second skew roller contacts a surface of a sheet at a position far from the reference surface than the centerline.
Recent image forming apparatuses are required to perform image formation on various types of sheets which are different from commonly used plain papers and coated papers in grammage, coefficient of friction, size, and conveyance orientation. According to the above-described conventional sheet conveyance apparatus, when the upstream skew roller obliquely conveys a sheet toward a reference surface, the sheet cannot stably maintain its orientation until the sheet reaches the downstream skew roller.
Although more details will be described in comparative examples, while only the upstream skew roller nips a conveyed sheet, the sheet rotates around the upstream skew roller. For example, if the lower surface of a lightweight sheet has a large coefficient of friction and the sheet is short in the direction parallel to a reference surface, the sheet rotates with a large angle and the amount of skew becomes larger.
When the amount of skew is excessively large, the downstream skew roller positioned near the reference surface may not be able to accomplish skew correction before the sheet thoroughly passes the reference surface. In other words, if a skew roller rotates a sheet, the rotating sheet cannot smoothly move toward the reference surface as intended and rather makes it difficult to accomplish the skew correction.
Furthermore, it is desirable that skew rollers do not interfere with a sheet during the skew correction or do not buckle the sheet in the process of aligning the sheet along the reference surface. To this end, when skew rollers obliquely convey a sheet, the rollers allow the sheet to freely slide and rotate. In a state where a sheet is obliquely conveyed by a pair of (upstream and downstream) skew rollers, if two rollers have differences in conveyance resistance (friction) acting between a sheet and a guide surface, the sheet rotates unwontedly and the orientation of the conveyed sheet becomes unstable.